1. Field of the Invention
The present invention relates to an apparatus for detecting imbalance abnormality in an air-fuel ratio between cylinders in a multi-cylinder internal combustion engine, and particularly, to those that can be suitably applied to an internal combustion engine having a plurality of cylinder groups.
2. Description of the Related Art
In general, in an internal combustion engine equipped with an exhaust purifying system using a catalyst, for highly efficiently performing purification of harmful substances in an exhaust gas by the catalyst, it is fundamental to control a mixing ratio of air and fuel in a mixture to be burned in the internal combustion engine, that is, an air-fuel ratio. For controlling such an air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the internal combustion engine, and feedback control is performed in such a manner as to make the air-fuel ratio detected by the air-fuel ratio sensor be equal to a predetermined target air-fuel ratio.
On the other hand, since the air-fuel ratio control is usually performed applying the same control amount to each of all the cylinders or each bank in a multi-cylinder internal combustion engine, an actual air-fuel ratio may vary between cylinders even if the air-fuel ratio control is performed. When the degree of the imbalance is small at this time, the imbalance can be absorbed by the air-fuel ratio feedback control and the harmful substances in the exhaust gas can be purified also in the catalyst, and the imbalance has no adverse influence on exhaust emissions and raises no particular problem.
However, when the air-fuel ratio varies largely between the cylinders due to a failure of a fuel injection system in a part of the cylinders, the exhaust emission is deteriorated, thus raising a problem. It is desirable to detect the imbalance in the air-fuel ratio as large as to thus deteriorate the exhaust emission, regarding it as imbalance abnormality. Particularly in a case of an internal combustion engine for an automobile, for beforehand preventing a travel of a vehicle in which the exhaust emission has deteriorated, it is requested to detect the imbalance abnormality in the air-fuel ratio between the cylinders on board (so-called OBD; On-Board Diagnostics), and there is recently a movement of legalizing such on-board detection.
For example, in an apparatus described in Japanese Patent Laid-Open No. 2010-112244, a variation parameter representative of the degree of unevenness in variations of a rotation speed of an output shaft in an internal combustion engine is detected, and when it exceeds a predetermined reference value, it is determined that abnormality occurs. Examples of the variation parameter include a rotation speed of the output shaft or a value as a difference in time required for rotation of a predetermined crank angle between neighboring cylinders in ignition order.
In an apparatus described in Japanese Patent Laid-Open No. 2013-011246, a difference in a variation parameter between at least one set of opposing cylinders that are different by 360 degrees in ignition timing from each other is used to determine imbalance abnormality. According to this configuration, it is possible to restrict a measurement error due to product variations in a timing rotor fixed on an output shaft (crankshaft), particularly due to variations in a rotational position of a number of projections formed on a timing rotor peripheral surface.
Incidentally in the internal combustion engine having a plurality of banks as in the case of Japanese Patent Laid-Open No. 2013-011246, even if variations occur in the rotation speed of the output shaft between the opposing cylinders that are different by 360 degrees in ignition timing from each other, in a case where a rotation speed of the output shaft in each cylinder inside each of banks in which the opposing cylinders are disposed is balanced, even in a case where air-fuel ratio feedback control is performed in each bank, an air-fuel ratio of each cylinder in the bank does not deviate largely from a target value, so that deterioration of exhaust emissions is not generated substantially. However, although the deterioration of the exhaust emissions is not substantially generated in such a case, due to variations that occur in the rotation speed of the output shaft between the opposing cylinders that are different by 360 degrees in ignition timing from each other, the variation results in being detected as abnormality.
Therefore, an object of the present invention is to provide an apparatus for detecting imbalance abnormality in an air-fuel ratio between the cylinders in a multi-cylinder internal combustion engine provided with a plurality of cylinder groups configured with a plurality of the cylinders, comprising an imbalance determining unit configured to determine imbalance of an air-fuel ratio of a first cylinder belonging to a cylinder group based upon a difference value between an index value correlative with a crank angular speed detected in the first cylinder and an index value correlative with a crank angular speed detected in a second cylinder belonging to another cylinder group, for restricting determination of the imbalance abnormality in a case where a torque difference exists between the cylinder groups but an index value of each cylinder inside the same cylinder group is equalized.